Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to a collimator assembly having variable pitch collimator plates.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator assembly for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
The post patient x-ray collimator used in CT detection is a device mainly made of a highly absorbing material such as tungsten or molybdenum plates or high-Z material alloy aligned to a focal spot on the x-ray tube. The main function of the collimator is to select x-rays along a particular direction (primary beam from focal spot) and to reject scattered radiation from other directions (patient scattered radiation). For this purpose, collimating plates are placed in front of the scintillator pixels to eliminate scattered radiation from the patient. In one example, a collimator includes tungsten or molybdenum plates placed in front of the interfaces of the detector cells (detector septa), requiring high precision manufacturing and alignment/precision features within the individual parts for alignment purpose.
Typically, the pitch of the collimator plates is the same as the pitch of the scintillator pack (array) channels. However, because the pack is flat and does not follow a curved surface like the collimator, error may be accumulated from a misalignment point of view between the scintillator pack and the collimator. In this case, for a collimator having a collimator plate aligned with the septum between the center channels of a scintillator pack, the error will increase from the center of the scintillator pack toward the edge channels for each scintillator pack. The increasing error causes varying spectral performance and gain response as a function of focal spot motion of the scintillator pack channels.
Therefore, it would be desirable to design a collimator that improves the spectral performance and sensitivity of the scintillator pack channels.